The invention generally relates to residual fuel dissipation for a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).
A fuel cell system may automatically or manually be shut down for purposes of repairing the system or for performing routine maintenance on the system. However, such a shut down may pose problems due to the residual fuel that is left in the fuel cell stack after the shut down. For example, the residual fuel is effectively potential energy that may deliver an electrical shock to a technician attempting to service the system. As another example of the problems posed by the residual fuel, the residual fuel may not satisfy the appropriate stoichiometric ratios and thus, may cause some of the cells of the fuel cell stack to exhibit negative voltages and enter unstable and potentially unsafe states in which these cells may produce hydrogen on the wrong side of the cells.
Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are recited above.
In an embodiment of the invention, an apparatus includes a fuel cell subsystem, a first circuit and a second circuit. The fuel cell subsystem produces power for a load in response to receiving a flow of a reactant, and the first circuit is coupled to the fuel cell subsystem to halt the flow to the fuel cell stack and isolate the fuel cell stack from the load after the flow is halted. The second circuit monitors a characteristic of the fuel cell subsystem and receives power from the stack to dissipate a portion of the reactant that remains in the stack while the flow is halted.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.